The present invention relates to addition cure polysiloxane compositions with vinyl gum cure accelerators which are particularly well suited for coating applications and are especially useful in the manufacture of paper and other articles having release coatings.
Silicone compositions have long been used for rendering surfaces nonadherent to materials which would normally adhere thereto. For a long time, it was necessary that these silicone coatings be applied as a dispersion within a solvent in order to control the viscosity of the coating material so as to be suitable for coating applications. However, although the solvent aids in the application of the coating, it is a highly inefficient process inasmuch as the solvent must thereafter be evaporated. The evaporation of solvents requires large expenditures of energy and pollution control requirements mandate that solvent vapors be prevented from escaping into the air. Removal and recovery of all the solvent entail considerable expenditure for apparatus and energy.
Thus, it has been noted that there is a need to provide a solventless coating composition which will, however, remain easy to apply to the substrate. Such solventless coating compositions are sometimes referred to as "100% solids" compositions. The absence of solvent in such compositions both lowers the amount of energy required to effect a cure and eliminates the need for expensive pollution abatement equipment. It is noted that even previous solventless coatings are limited to certain applications because oven cure temperatures must still be in the neighborhood of 300.degree. F. minimum, at which temperature many substrates are thermally degraded. Although cure time can be shortened with solventless grades, high temperatures can require that only high-density paper or high-melting point plastics and the like be utilized, so that the substrate will not char, melt or otherwise degrade during cure. The composition of the present invention is a low viscosity polydiorganosiloxane fluid which can readily be coated onto a substrate with or without solvent by standard methods such as blade, press, brush or gravure and which will thermally cure onto the substrate to form a nonadherent surface at lower temperatures than previously necessary.
Silicone release coating compositions made in accordance with U.S. Pat. No. 4,256,870 (Eckberg) can be cured at temperatures as low as 200.degree. F. thereby enabling coating upon a wider range of substrates than formerly possible. However, to effect an acceptable cure at reduced temperatures a large amount of platinum catalyst is required. For example, as much as eight times the amount of platinum might be necessary for low temperature cure compared to the composition in the Eckberg patent undergoing conventional high temperature cure.
Since the high platinum concentration (e.g. 200 ppm instead of 25 ppm) adds a large cost to such grades, there was considerable interest in efforts to limit the level of required platinum without sacrificing low temperature cure performance. As a result of the present invention, it has been discovered that certain vinyl functional siloxane gums will act as cure accelerators allowing lower temperature cure with levels of platinum much lower than would be required in the absence of the vinyl gums.
Release coatings are useful for many applications whenever it is necessary to provide a surface or material which is relatively nonadherent to other materials which would normally adhere thereto. Silicone paper release compositions are widely used as coatings which release pressure-sensitive adhesives for labels, decorative laminates, transfer tapes, etc. Silicone release coatings on paper, polyethylene, Mylar, and other such substrates are also useful as non-stick surfaces for food handling and industrial packaging applications.
For example, when labels are coated with an adhesive it is desirable that the paper backing be easily peeled away from the label when it is ready for use, yet the adhesive quality of the label should not be derogated by the fact that it has been peeled away from the substrate upon which it was stored. The same principle applies to certain types of adhesive tapes which come in rolls. It is necessary that the tape unroll easily and still maintain its adhesive characteristics. This can be accomplished by coating the nonadhesive side of the tape with a silicone release composition which will come into contact with the adhesive as the roll of tape is manufactured.
Two other problems previously encountered with silicone compositions of the above-described type are related to premature gelation of the composition. Inasmuch as a typical composition can be made up of a vinyl chainstopped base polymer, and a platinum metal catalyst, premature curing may occur over a length of time resulting in a product with unsatisfactory shelf-life. Furthermore, when the composition is ready for use, at which time a reactive crosslinking agent is introduced, it is desirable that the catalyzed polymer composition have a sufficiently long pot-life to enable one skilled in the art to carry out the coating application.
These additional problems were solved in U.S. Pat. No. 4,256,870 (Eckberg) which is hereby incorporated by reference, which provided solventless addition cure silicone release coating compositions which contained effective inhibitor components which provided stable products. These compositions were thermally cured at elevated temperatures to obtain release coatings. As noted, it is necessary that increased amounts of precious metal catalysts such as platinum or rhodium be utilized in order to effectively lower the cure temperature of these coatings, but at significantly increased cost.
Therefore, a primary objective of the present invention is to provide a solventless composition which will render surfaces nonadherent to materials which would normally adhere thereto.
Another object of the present invention is to provide a solventless coating composition with a satisfactory shelf-life which will cure at relatively low temperatures.
Another object of the present invention is to provide a solventless coating composition utilizing novel vinyl siloxane gum cure accelerators.
Another object of the present invention is to provide a method of rendering materials nonadherent to other materials which would normally adhere thereto.
Another object is to provide articles of manufacture having nonadherent surfaces by coating the articles with the release compositions of the present invention.
Another object of the present invention is to provide a solventless silicone composition which is particularly well suited for paper release applications.